Almost every single non-theoretical problem that blockchains solve is something we've already solved. And most of the problems you could solve with a blockchains are severely limited by data-size limitations.

It would be amazing if I could decentrally store, say, a movie or videogame on a blockchain. Then, I could sell access tokens, would the owners could resell as they wanted. That's a GREAT way to use blockchain tech, because people would always have access, and they could use or sell the keys as they wanted. It doens't work though, because in the real world, that movie doesn't fit on the blockchain, it'll just be a link the a secondary source, and the whole thing falls apart.

And that's really the problem. Blockchains have a lot of nifty uses, but it almost always immediately falls apart around the edges, where it touches on non-blockchain tech, or, even worse, physical objects.